Paper manufacture



P wn e 19 Georgob. Brown ofllalne '1 No serliv-llfllo. 13,9

Co m. Ber

munracruu ll'ichteaherlin, N. 11., assignor to- N. 11., a corporation(cl. ill-68) This invention relates to the manufacture of papers havingthe cofijunctive properties of high absorbency and high wet-strength.When paper is put to such .uses as toweling.

napkins, diaper linings; handkerchiefs, etc., it

should preferably benot only of high absorbency but. also of sufficientwet-strength to resist the disintegrating action of the water or aqueousmedium absorbed thereby. Within recent years, it has been discoveredthat an absorptive paper base can be made -to acquire high wet-strengthwhile substantially retaining its absorptive capacity by thinlyenveloping substantially all of its fibers with regenerated celluloseproduced in situ in thebase from viscose or other'cellulose derivativeand serving to bind together the fibers. It was then discovered that analbuminous adhesive, such as 'gluefalthough of radically differentchemical composition from regenerated cellulose, functions in a waysimilar to regenerated cellulose provided that the albumen is in-.'solubilized or tanned by formaldehyde or its equivalent. Both'regenerated cellulose and glue, however, have the common property ofbeing water-absorbing to a considerable-extent; and it is evidently thisproperty that permits water to difiuse or penetrate through the thinenvelops or coatings of binder to the fibers of the paper base while atthe same time performing the role of binding together the. fibers quitestrongly and thus of resisting fiber-separation under manipulation thatwould bespeak rupture of the base lacking the binder-reinforcement.

I have now found that despite the fact that.

cellulose acetate is decidedly diflerent from both regenerated celluloseand glue in that it is water repellent, it can be incorporated to goodad- 'vantage' as a wet-strengthening agent into a paper I base to yielda paper product of high water-ab- '40 sorptivity and high wet-strengthand hence especially .adapted for such uses as have hereinbefore' beenmentioned. When I say that cellulose acetate is water-repellent, I meanthat, unlike a dried glue film, which swells appreciably when soaked inwater, a dried cellulose acetate film formed from an organicsolventsolution of cellubase of high p rosity and absorbency and to in-7 corporate the cellulose acetate 'thereinto in a state of attenuationso great that the pore or void space in the base is substantially'thesame as that .55

hazard than ordinary papers.

it has'undergone. Thus, a refined wood pulp in a similar base lackingthe acetate. This means the water-holding capacity m: the base issubstantially unaflfected by the cellulose acetate, whereas thewet-strength of the base is greatly in-- creased by reason of thebonding action of the cellulose acetate on the flbera- The resulting 5paper products are substantially odorless. and non-toxic. Because of thepresence therein of the cellulose acetate, they present even less fireIf desired, the paper product of the present invention may be softenedas by being led into frictional or rubbi contact with the edge of adoctor blade or by being impregnated with such cellulose-softenin orplasticizing agents as glycerine, ethylene glycol, oils, or the like.

In producing a paper product embodying the present invention for suchpurposes, for instance, as toweling, the procedure may be as follows:-.The absorptive paper base may be fabricated from substantially unbeatenor only lightly beaten 20 papermaking stock on a high-speed papermak-'ing machine, 'as usual; and the paper may be creped while still on themachine as ordinarilf and collected in rolls as it is being delivered bythe machine. The paper base thus fabricated 25 may have pore or voidspace therein amounting to, say, or more ofthe space occupied by thesolid fibers alone. In other words, the waterholding capacity of thepaper may amount roughly to 100% or more, by weight of the dry fibers,the particular water-holding capacity de-' pending upon the cellulosepulp used as raw material and the conditioning treatment, for in stance,the degree of preliminary beating, that consisting essentially of alphacellulose, such as I may advantageously employ as raw material,

.wouldunder given conditioning treatment lead to a more absorptive paperbase than such ordinary pulps as kraft and/or sulphite, such asI mayalso employ. The dried creped paper base may be progressively withdrawnfrom a roll and passed progressively through a bath of cellulose acetatesolution containing any one of the usual volatile cellulose acetatesolvents, such as acetone, ethyl acetate, etc. It is necessary that thecellulose acetate solution used as the impregnant for the paper base bequite dilute in order to deposit cellulose acetate in the desired highlyattenuated state in the paper body, that is, without detractingsignificantly from its void or interstitial space. To this end, thecellulose acetate solution constituting the bath is preferably one whosecellulose acetate concentration or strength ranges from 0.5 to 2%. Sucha solution containing,

limited small amount of cellulose acetate dis-' for instance, acetone asthe solvent, is quite thin and fluent and so penetrates the paper basequickly and substantially uniformly throughout. As the impregnated paperbase leaves the bath of cellulose acetate solution, free or excesssolution may be removed therefrom, as by the action of squeeze rolls, toleave only the desired tributed substantially uniformly through thepaper body. It is preferable that only about 0.5 to 2.0% of celluloseacetate, based on the dry weight of fiber, remain in the paper body inorder to impart thereto the desired characteristics.

The particular amount of cellulose acetate with-- in the foregoing rangethat may be incorporated into the paper base for the purpose of thepresent invention. depends upon the compactness of the The impregnatedpaper sheet may then be dried as by passage through a hot air chamber orover drier drums, the particular dryer employed preferably beingequipped with a solvent-recovery system. The cause of the comparativelysmall amount of solvent present in the impregnated paper and thevolatility of the solvent, drying of the impregnated paper may beeffected rapidly, insomuch that it is possible to employ in. acontinuous process, such as described, only one large drier drum such asconstitutes the dry end of a so-called Yankee papermaking machine.

. the Oils such as castor oil do like- If desired, the dilute celluloseacetate solution may be applied to the paper base other than byimmersion of the base therein. Thus, the' solution may be spread on aface of the base or be picked up by the base from an applicator orkissing roll rotating partially submerged in the solution, in which casethe solution because of its high fluidity penetrates quickly andsubstantially uniformly throughout the paper body. The amount ofcellulose acetate thus introduced into the base is controlled as in theimmersion and squeezing treatments already described.

. The dried impregnated product may be softened, if desired, ashereinbefore described. when softening agents are used, however,thermal?- be added to the cellulose acetate solution so as to obviate. aseparate "softening treatment. For

instance, rather than glycerinating' the dried impresnated product,'glycerine may. be added in appropriate amount to the impregnatingcellulose acetate solution, particularly when the cellulose acetatesolvent is acetone, since glycerine is conipatible with acetone. cresylphosphate, dibutyl phthalate, etc. may also be added tothe celluloseacetate solution to develop soft or emollient quality in-the finishedpaper product. These latter plasticizers act as softeners upon both thecellulose acetate and The rate at which water is or imbibed by thefinished paper product may be increased by adding small amounts ofsuitable so-called wetting-out agents to the paper base or to theimcellulose acetate solution. Among such agents may be mentioned NekalBX,

.for the crepe to pull out.

Such plasticizers as triwhich is the sodium salt of naphthalenesulphonic acid with side chains; Nekal A", which is the crude sodiumsalt of octohydro-anthracene metasulphonic acid; "Nekal AEM, which is acombination of Nekal A with methylhexalin; "Neomerpin N", which is ahigh molecular weight sulphonic acid; etc.

The finished impregnated paper product has substantially the creped formimparted to the paper base during the papermaking operation by reason ofthe fact that the organic solvent solution of cellulose acetate does nothydrate or swell the fiber as does water or an aqueous solution and,accordingly, there is very little tendency Since the cellulose acetatesolution is colorless, a white paper base treated as hereinbeforedescribed is not dis-- colored; nor does the cellulose acetate undergoany decomposition necessitating any chemical after-treatment of theimpregnated paper product.

The absorptivityof the finished paper product may be increased bycausing a precipitation of the cellulose acetate inthe freshlyimpregnated paper base. Thus, rather than passing the paper baseimmediately after impregnation with the dilute cellulose acetatesolution to a dryer,the wet, impregnated base may be treated with water,as by running it through a water bath, to cause the acetate'toprecipitate out of solution onto the fibers, whereupon excess water maybe removed from the sheet as by squeeze rolls and the sheet then dried.Such a practice works to good advantage when the cellulose acetatesolvent is, like acetone, ,water-miscible. The cellulose acetate solvent,leached out of the sheetby the water bath may, when the bath becomesoverconcentrated therewith, be recovered by distillation from the batheilluent. In such case, the fibers of the finished paper product carryminute discrete particles of cellulose acetate which perform afiber-bonding service but do not prevent quick access of water to theindividual fibers themselves.

Other water-repellent cellulose esters might be used in lieu ofcellulose acetate according to-the present invention, but celluloseacetate is the preferable ester, possessing the advantage of lower costover such esters-as the formate and butyrate and being 'without the fireand ex-- plo'sion hazard of the less expensive cellulose nitrate. I

It is a surprising phenomenon that despite their contentofwater-repellent material, the paper products of the present inventionhave high water-holding ability. In other words, they do not tend toshed water as do so-called sized papers. While I cannot account withprecision for the water-holding ability of. the paper products of thepresent invention, it is my theory that, unlike the initiallywater-soluble wet-strengthening agents, namely, the regeneratedcellulose and the glue, hereinbefore mentioned, which are believed toattach themselves'tightly on the fiber walls and to wet all of the wallsurface but nevertheless to permit water to diffuse therethrough to thefiber walls, the cellulose esters by virtue of 1 deposition from organicsolvent solutions do not become so intimately attached to the fiberwalls as not .to permit seepage in between them and the fiber walls andfurther do not "wet orcover all the fiber surfaces. In other words, itmaybe the case that even though practically all of the fiber surface isthinly coated with the waterrepellent cellulose ester, there aremyriadmicroscopic loci on the fiber walls that have escaped wetting by theester and that constitute startin points for the water to hold onto thepaper and to spread by capillary action into the bodies of the fibers aswell as to stay in the interstitial or void space of the paper body.This does not happen when a paper base is treated with a comparativelystrong solution of cellulose ester; and in terms of my theory this couldnot happen, since the amount of cellulose ester available for coatingthe fiber walls and occupying the pores and interstices of the paper isso large as leave no uncoated starting points on the wall surfaces forthe influx of water into and throughout the fiber bodies or walls. I

It might be remarked that when a cellulose acetate film is cast or setfrom an acetone solution, it is necessary to take certain precautions inorder to avoid what is sometimes termed a milky or smoky effect in thefilm. Indeed, it

is quite difilcult to produce from acetone solutions of celluloseacetate films which are per.- fectly transparent. The presence of smallamounts of water in the solvent is responsible for impairment in thetransparency of the film; and, unless precaution is taken to excludewater-from the acetone solvent, residual traces of water in the filmwill spoil its transparency. Undoubtedly, the acetone evaporates morerapidly than the water and as the last traces of water are expelled fromthe cellulose acetate film, there is a crazing action on the film, whichis very apparent. So far as I have been able to ascertain, when a verydilute cellulose acetate solution-is applied to a porous paper sheet, ashereinbefore described, and when the solvent, for instance, the acetone,is evaporated, there-is no continuous film of acetate deposited in thesheet but only multitudinous partly connected cellulose acetateparticles between which there is a definite lack of the kind ofcontinuity that exists in a cellulose acetate film. This is particularlytrue if the paper base is treated with the dilute cellulose acetatesolution in the presence of a small amount of moisture, that is, whenthe base sheet carries moisture therein and/or when the celluloseacetate solution itself is notcompletely anhydrous but carries a smallamount of water dispersed or dissolved therein. Accordingly, it may bedistinctly desirable in accordance with the present invention to treatthe paper base while it contains a small percentage of moisture, say,about 5 to 12% of moisture and/or to use as the treating solution adilute cellulose acetate solution containing, say, about 2% to 5% ofwater;

dissolved or dispersed inthe volatile organic solvent solution, forinstance, an acetone solution of cellulose acetate containing suchpercentage of water dissolved therein.

. prises impregnating I claim:

1. A water-absorbing paper product of high wet-strength comprising ahighly porous paper body and containing substantially uniformlydistributed therethrough a water-repellent cellulose ester in the amountof about 0.5%, to 2.0%, based on the dry weight of the paper body,deposited in said body from organic solvent solution in an attenuatedand discontinuous such that water brought into contact with said paperproduct spreads by capillary action to the bodies of the fibers as wellas entering the pores and interstices in said paper body.

2. A water-absorbing paper product of high wet-strength comprising ahighly porous body and containing substantially uniformly distributedtherethrough cellulose acetate in the amount of about 0.5% to 2.0%,based on the dry weight of the paper body, deposited in said paper bodyfrom organic solvent solution in an attenuated and discontinuous filmsuchthat water brought into contact with said paper product spreads bycapillary action to the bodies of the fibers as well as entering thepores and interstices in said paper body.

3. A method of producing a water-absorbing paper product of highwet-strength, which comprises impregnating a highly porous paper bodysubstantially unif rmly throughout with a dilute organic solventsolution of a water-repellent cellulose ester in such amount as toincorporate into said body about 0.5%to 2.0% of said cellulose ester,based on thedry weight of said paper body, and removing the organicsolvent from the impregnated paper body, and depositing saidcelluloseester therein from solution in an attenuated and discontinuousfilm such that water brought into contact with the resulting paperproduct spreads by capillary action to the bodies of the fibers as wellas entering the pores and interstices in'said paper body.

4. A method of producing a paper product of high wet-strength, whichcoma highly porous paper body substantially. uniformly throughout with adilute organic solvent solution of cellulose acetate in such amount asto incorporate into said body about 0.5% to 2.0% of cellulose acetate,based on the dry weight 01' said paper body, and nemoving the organicsolvent from the impreg GEORGE a. RICIHI'ER.

the pores and interwater-absorbing

